The present invention relates to a scanning optical system that is used as an optical system for a scanning optical device such as a laser beam printer.
The scanning optical device deflects a beam emitted from a light source such as a semiconductor laser by means of, for example, a polygonal mirror, and converges the beam to form a beam spot on an object surface to be scanned such as a surface of a photoconductive drum, through an fxcex8 lens (i.e., a scanning lens). The beam spot formed on the object surface moves (i.e., scans) on the object surface in a predetermined scanning direction as the polygonal mirror rotates.
In this specification, a scanning direction of the beam spot on the object surface is referred to as a xe2x80x9cmain scanning directionxe2x80x9d, a direction perpendicular to the main scanning direction on the object surface is referred to as an xe2x80x9cauxiliary scanning directionxe2x80x9d. Shapes and orientations of powers of respective optical elements will be defined on the basis of these scanning directions. Further a plane in which the optical axis of the scanning lens is located and is perpendicular to the rotation axis of the deflector is referred to as a xe2x80x9cmain scanning planexe2x80x9d.
In such a scanning optical system, lateral chromatic aberration should be corrected to reduce a variation of printing performance among the systems due to an individual difference of a emission wavelength of a semiconductor laser. Further, the correction of the lateral chromatic aberration is absolutely necessary for a multi-beam scanning optical system, which employs a plurality of laser sources to form a plurality of scanning lines per one scan, in order to compensate for a variation of emission wavelength among laser sources of the system.
Conventionally, the chromatic aberration of the fxcex8 lens is corrected by combining a positive lens and a negative lens having different dispersion. In order to correct the chromatic aberration of the fxcex8 lens by selecting lens materials (glass materials) having different dispersion as in the prior art described above, the number of lens elements of the fxcex8 lens increases as compared with a case where the chromatic aberration is not corrected. In addition, in order to correct the chromatic aberration, lens materials cannot be selected only by their refractive indexes, and types of available lens materials are limited, thereby degree of freedom in designing the lens is lowered.
It is also known as a prior art that lateral chromatic aberration is corrected by means of a combination of a refractive lens and a diffractive element. For example, Japanese Patent Provisional Publication No. Hei 11-095145 discloses a scanning optical system that employs the diffractive element located between a polygon mirror and an fxcex8 lens to correct the lateral chromatic aberration caused by the fxcex8 lens. Further, the publication indicates that the diffractive element doubles as a cover glass of a noise reduction cover for the polygon mirror.
When a center axis of the laser beam incident on the polygon mirror and the optical axis of the fxcex8 lens are located in the same plane and cross each other at a predetermined angle, the noise reduction cover must allow to pass both of the incident laser beam onto the polygon mirror and the reflected laser beam from the polygon mirror at different areas.
However, since the diffractive element disclosed in the Publication is arranged to be perpendicular to the optical axis of the fxcex8 lens, another cover glass through which the incident beam onto the polygon mirror passes is required in addition to the diffractive element that doubles as the cover glass through which the reflected beam from the polygon mirror passes. Further, unnecessary light reflected by the diffractive element tends to be incident on the object surface as ghost light.
On the other hand, when the diffractive element is inclined with respect to the optical axis of the fxcex8 lens, the diffractive element allows to pass the incident laser beam onto the polygon mirror and the reflected laser beam from the polygon mirror. That is, the other cover glass in addition to the diffractive element is unnecessary. However, in this case, the lateral chromatic aberration becomes larger relative to the case where the diffractive element is perpendicular to the optical axis.
It is therefore an object of the invention to provide an improved scanning optical system that is capable of reducing the lateral chromatic aberration when the diffractive element is inclined with respect to the optical axis of a scanning lens.
For the above object, according to a first aspect of the invention, there is provided a scanning optical system, including a light source; a deflector, which deflects a beam emitted from the light source; a scanning lens having positive refractive power for converging the beam deflected by the deflector onto an object surface to be scanned; and a diffractive element, which is located between the deflector and the object surface, for correcting chromatic aberration caused by the refractive power of the scanning lens. The diffractive element employs a diffractive surface that is formed to be symmetrical with respect to a predetermined reference point in the main scanning direction. The diffractive element is arranged such that the normal of the diffractive surface at the reference point is inclined with respect to the optical axis of the scanning lens in the main scanning direction and that the reference point deviates from the optical axis in the main scanning direction.
With this construction, the variation of the lateral chromatic aberration is averaged, which can lower the maximum value thereof even if the diffractive element is inclined with respect to the optical axis of the scanning lens.
The diffractive element may be a flat plate in macroscopic view having almost no power in proximity to the reference point. Further, the diffractive element may be located between the deflector and the scanning lens. In such a case, one edge of the diffractive element is close to the deflector and the other edge is apart from the deflector in the main scanning direction relative to a reference condition where the normal of the diffractive surface at the reference point is parallel to the optical axis of the scanning lens. Further, the reference point should deviate from the optical axis toward the edge that is apart from the deflector.
When the center axis of the laser beam incident on the deflector and the optical axis of the scanning lens are located in the main scanning plane and cross each other at a predetermined angle, the laser beam is incident on the deflector from outside the scanning area on one side of the optical axis of the scanning lens. In this case, when the diffractive element is inclined such that the light source side edge thereof is close to the deflector, the reference point should deviate from the optical axis in the direction away from the light source. On the contrary, when the diffractive element is inclined such that the light source side edge thereof is apart from the deflector, the reference point should deviate from the optical axis in the direction nearer to the light source.
Assuming that an inclination angle xcex8 (unit: degree) of the normal of the diffractive surface with respect to the optical axis has a positive value when the diffractive element is inclined such that the light source side edge thereof is close to the deflector, the following condition (1) is preferably satisfied when xcex8≳0, and the condition (2) is preferably satisfied when xcex8 less than 0;
(1) 0 less than S less than 0.7xc3x97|xcex8|
(2) xe2x88x920.7xc3x97|xcex8| less than S less than 0
where S (unit: mm) is a deviation amount of the reference point with respect to the optical axis of the scanning lens and has a positive value when the reference point deviates in the direction away from the light source.
According to a second aspect of the invention, there is provided a scanning optical system, including a light source; a deflector; a scanning lens; and a diffractive element, which is located between the deflector and the object surface, having a diffractive surface to correct chromatic aberration caused by the refractive power of the scanning lens. The diffractive element is arranged such that the normal of the diffractive surface at an intersection point with the optical axis of the scanning lens is inclined with respect to the optical axis in the main scanning direction. Further, an additional optical path length added by the diffractive surface asymmetrically varies with the distance from the optical axis in the main scanning direction and the additional optical path length has the minimum value at the intersection point.
With this construction, at least a part of an asymmetry of the lateral chromatic aberration caused when the diffractive element is inclined with respect to the optical axis of the scanning lens can be counterbalanced with the asymmetrical variation of the additional optical path length added by the diffractive surface, which can reduce the lateral chromatic aberration.
The diffractive element may be a flat plate in macroscopic view having almost no power in proximity to the intersection point.
The additional optical path length added by the diffractive surface is expressed by the following optical path difference function "PHgr"(Y):
"PHgr"(Y)=P1Y+P2Y2+P3Y3+P4Y4+P5Y5+P6Y6+. . . 
where Pn are coefficients of n-th order, Y is a distance from the optical axis in the main scanning direction and xcex is wavelength. When the center axis of the laser beam incident on the deflector and the optical axis of the scanning lens are located in the main scanning plane and cross each other at a predetermined angle, the first order coefficient P1 satisfies the condition (3) when the diffractive element is inclined such that the light source side edge thereof is close to the deflector, or satisfies the condition (4) when the diffractive element is inclined such that the light source side edge thereof is apart from the deflector.
(3) P1≳0
(4) P1 less than 0
The macroscopic shape of the diffractive surface may be an anamorphic surface whose rotation axis is parallel to the main scanning direction.